FR2791663A1 - METHOD FOR MANUFACTURING A REFORM FOR OPTICAL FIBER AND MORE PARTICULARLY A PREFORM OF HIGH DIAMETER - Google Patents

Abstract

An optical fiber preform overcladding process, comprising intermediate unilateral layer length reduction to obtain reduced material thickness on one of the preform holder tips and a limited preform length, is new. An optical fiber preform is produced by subjecting a preform (1'), held horizontally at its ends between two mounting point support tips (6a, b), to axial rotation and repeated displacement relative to a plasma torch and an associated a material feeder deposition of a new material layer on the preform or smoothing of the last deposited layer being carried out during the relative preform/torch displacements. Unilateral layer length reduction of one of the intermediate layers is carried out in one of the passes, during the deposition of a sequence of concentric material layers of respective lengths which are progressively reduced progressive reduction of the displacement length so that the thickness of deposited material, which covers the preform and parts the tips, is reduced regularly towards the ends. The unilateral layer length reduction leads to a reduced material thickness, determined by the layer deposited prior to the unilateral layer length reduction, on one of the tips and on a preform zone of limited length.

Description

Method for manufacturing a preform for optical fiber and more

  particularly of a large diameter preform The invention relates to a method of manufacturing preforms for optical fiber which is more particularly intended for the manufacture of large diameter preforms. A known method of manufacturing, or recharging, preforms intended for the production of optical fibers provides for a deposition of silica on a primary preform which is provided with support ends at its ends and which is carried by means enabling it to move along its axis and rotate relative to a flame from an inductive plasma torch, into which grains of silica are injected and melted. This process makes it possible to produce a preform of determined thickness, starting from a thinner primary preform, by superposition of a succession of layers of concentrically produced silica. The respective lengths of these layers are decreasing so that the thickness of deposited silica, which covers the preform and its tips, is reduced regularly towards the ends, from a central section of determined length and diameter. A conical shape is given at one end of each preform, in order to facilitate the subsequent operations of

  fiberizing, from this end.

  It is known to obtain the taper, which is sought at the end of a preform fiber drawing, by drawing carried out by pulling in opposite directions on the tips of the preform, recharged with silica, after the latter has been locally melted. by central heating, in a zone of the central section located near the end to be made conical. This stretching allows the preform to be separated from one of its two support ends. The intense heating carried out with a view to drawing has the drawback of causing strong evaporation of silica, the cooling of which leads to the formation of soot which in particular falls on the preform. Now these

  soot affects the transparency of the preform and increases its roughness.

  To remedy these drawbacks, document EP-A-0831070 of the applicant plans to carry out the separation in two stages. A first stretching step reduces the diameter of the preform in the heated zone for sectioning, up to a chosen diameter which is usually close to the diameter of a support nozzle. A glazing operation is planned to remove the parasitic deposit which has occurred on the cold parts of the preform having received soot during the heating carried out for the first stretching step. This icing is carried out by passing the preform through the flame of the plasma torch, without any material being added. A second stretch completes the separation in the area,

2 2791663

  smaller diameter, fully heated for this purpose. The quantities of silica, evaporated and

  redeposited, are reduced due to the small dimensions of the area then heated.

  This method providing for two separation stages is very suitable, when the preforms to be sectioned have a diameter which is not too large, for example a maximum diameter of the order of 80 mm. This is not the case, when the preform diameter is such that it makes the core heating long and difficult and therefore obtaining sufficient softening of the

  preform in the separation zone where stretching must be carried out.

  Furthermore, the prolonged heating of a large diameter preform causes significant evaporation of silica, from the heated zone, and leads to a thick redeposition, on the cold parts of the preform and in particular on that close to what will constitute the cone. This thick redeposition risks vitrifying badly, during the intermediate glazing operation, and leading to the appearance

  of an area which is not very homogeneous and which can disturb the fiber drawing.

  To overcome these drawbacks, the invention provides a method of manufacturing or refilling a preform for optical fiber in an installation provided with means allowing an axial rotation of a preform, held horizontally at the ends between two mounting points by end caps. support, and relative translations of the preform, held. Preform heating means by plasma torch, radially disposed relative to said preform are associated with material supply means, to allow manufacturing by successive passes corresponding to relative displacements preform / torch, with or without supply of material, leading consequently, either to a deposition of a new layer of material on the preform, or to a glaze on the last layer

filed.

  According to a characteristic of the invention, the method provides for inserting at least one unilateral reduction in layer length, during a pass and from one of the intermediate layers, during the deposition of a series. of concentric layers of material on the preform o the respective lengths of the layers, determined by the relative preform / torch displacements, are gradually reduced in length due to a progressive decrease in the length of the displacements, so that the thickness of material deposited, which covers the preform and partially the end pieces, is reduced regularly towards the ends, said unilateral reduction in layer length leading to a limitation of the thickness of material, deposited on one of the end pieces and for a preform area,

3 2791663

  limited in length, which adjoins this end piece, at the level fixed by the deposited layer

  prior to this unilateral reduction.

  According to a characteristic of the invention, the method comprises a hot drawing operation carried out, to ensure a separation between a preform and one of the end pieces in said preform area, limited in length, which adjoins this end piece,

  after completion of the series of layers necessary for the formation of the preform.

  According to a characteristic of a variant of the invention, the method comprises a hot stretching operation carried out in two stages, separated by a glaze operation of the preform, in said preform zone, limited in length, which adjoins a end piece to ensure separation between the preform and this end piece, the first stretching step being associated with core heating causing softening by fusion in said preform area, limited in length, and producing a reduction in diameter, the second step also associated with core heating resulting in melting softening and ensuring separation

complete.

  The invention, its characteristics and its advantages are specified in the

  description which follows in conjunction with the figures mentioned below.

  Figure 1 is a schematic representation of a known installation for implementing the method according to the invention for the production of a

preform.

  Figure 2 is a schematic representation of a preform made

according to the invention.

  Figure 3 is a schematic representation of a preform according to

  the invention as obtained after separation.

  The installation shown diagrammatically in FIG. 1 is supposed to allow the manufacture, or refill, of preforms for optical fiber, this operation being translated by the term "overcladding" in English. It makes it possible to obtain a recharged preform 1 from a primary preform 2, of axis 4, which is known in the art

  anterior and which is shown diagrammatically in FIG. 2.

  This installation comprises means 3, of the tower type, making it possible to ensure axial rotation of the preform held horizontally at its ends, by means of end pieces 6a, 6b, between two mounting points 3a, 3b. As is known, the tips have been previously fixed to the ends of the primary preform. One of them, here referenced 6a, is intended to remain integral with the preform finally obtained, while the other, here 6b, is intended to be separated from this final preform to allow obtaining a fiber optic by a

  fiberizing operation known to those skilled in the art.

  The installation also includes heating means by plasma torch which are most generally positioned radially with respect to the preform, when the latter is held horizontally between the mounting points 3a, 3b. Matter supply means, not shown, are associated with the plasma torch, they are conventionally used to inject grains of silica into the flame of the torch which ensures its fusion. The positioning of the torch is carried out in a known manner which ensures a localized deposit of silica on the preform. Relative translational movements between the preform and the torch make it possible to deposit silica over an area of limited width over the length of the preform and the rotation of the latter makes it possible to cover the entire preform by deposition. It is conceivable that the torch is moved in translation relative to the preform, but more conventionally the preform is mounted on a lathe and this ensures the displacements in translation of the preform, held by its ends, relative to

the torch then fixed.

  The refilling of a primary preform, such as 2 ′ in FIG. 2, in order to obtain a preform 1 or 1 ′ which can be used for the production of a fiber, is carried out by depositing, in successive passes, a series of concentric overlapping layers. It is known to progressively reduce the length of successive layers by gradually reducing the length of the relative preform / torch displacements, parallel to the preform axis 4. This makes it possible to obtain that the thickness of deposited material which covers the primary preform and partially the end pieces, is gradually reduced at the two ends and on either side of a central section of

  uniform diameter, as shown in Figure 1.

  It is necessary to separate one of the end pieces from the preform finally obtained, in order to obtain an end from which a fiber can be produced. This operation is carried out by stretching after intensive heating in a limited area of the preform where the glass is softened by fusion to facilitate separation. A strong evaporation of silica occurs during this operation and this silica falls back in the form of soot which solidifies on the parts

  the coldest of the preform where they are deposited.

  To remedy this drawback, document EP-A-083107 provides for a separation into two stages between which a glazing phase is inserted. The latter aims to eliminate the redeposition of silica which appeared during the first of the two separation stages. During this first step, the preform 1,

2791663

  fully recharged, is annularly reduced in diameter, to a value which corresponds, for example, to the diameter of a nozzle, in a determined area where the separation must be carried out. As shown in FIG. 1, this zone 10 is chosen close to the endpiece 6b to be eliminated and following part 9 of the preform constituted by the successive deposition of the layers of silica on the primary preform and on

  the end piece 6b, where they join before separation.

  The intensive heating carried out using the plasma torch 5, during the second stage which accompanies the separation carried out by stretching between the preform 1 and the nozzle 6b, only causes a limited evaporation of silica. This evaporation and the redeposition which corresponds to it are therefore significantly smaller than those occurring during the previous stage, insofar as the softening to be carried out only affects zone 10 and therefore only relates to a relatively small amount.

amount of silica.

  However, as indicated, this solution is not always fully satisfactory, in particular when the sectioning is to be carried out on a preform of

large diameter.

  According to the invention, an effort is therefore made to limit as much as possible the quantity of silica to be melted to the core and it is therefore planned to avoid as much as possible the deposition of concentric layers of silica on the preform 1 in the zones to be eliminated which

  correspond to zones 9 and 1 O, illustrated in FIG. 1.

  To this end, according to the invention, provision is made to insert at least one unilateral reduction in the length of the material deposition layer, during a pass and from one of the intermediate layers, during the depositing the series of concentric layers of material on the preform. The length of each concentric layer is a function of the length of relative displacement, provided between the torch in operation and the preform along the longitudinal axis 4 of this preform, to

the realization of this layer.

  A preform l 'obtained by the process according to the invention is produced from a usual primary preform 2', of cylindrical shape of revolution, carried at its ends by end pieces 6a 'and 6b', as shown in the figure 2. During a first part of the sequence of material deposition operations, the concentric layers are progressively reduced in length on either side of a central section, according to a law of variation which is for example linear and which is imposed the relative preform / torch displacements. Because of these reductions, the ends of the preform during recharging tend to form points taking

  each more or less approximately conical.

  A unilateral reduction in layer length is imposed by a reduction in the relative preform / torch relative displacement. It is produced on the occasion of a covering pass, by a layer of material, of the preform 1 ′ during recharging. This reduction is, for example, fixed at a value L1 between 10 and 200 millimeters. It is for example triggered when a determined value D1 of diameter is reached by the preform during recharging, due to the successive deposits of material during the passes, these having led to the formation of a predetermined number of layers . This diameter value Dl is chosen to be greater than the diameter of the endpiece 6b ′ and preferably less than 70

millimeters.

  The deposition of concentric layers is then continued on the preform 1 ′ starting from the unilaterally reduced layer and with a law of variation of decreasing length which is possibly the law of decrease previously exploited for the layers whose superposition has made it possible to reach 1 material thickness corresponding to the diameter Dl. This law of decrease is for example a linear law leading to the production of a trunk of revolution intermediate between the central cylindrical section of the recharged preform 1 'and the cylindrical section 1 0' of diameter Dl which was caused by the reduction unilateral in length made on the end of the end piece 6b '. It practically results in a series of control points, the characteristics of which are supplied to an automaton, not shown, which is responsible for controlling the preform / torch movements. The law of decrease is assumed to be the same for the two ends of the preform in the example illustrated on

Figure 2.

  A different law of decrease can also be envisaged, on the one hand, for each end of the preform and, on the other hand, for the intermediate part connecting the cylindrical central section of the preform to the cylindrical section 10 ′. This is to obtain a preform end whose shape differs slightly from the cone, after elimination of the tip 6b ', for example by approaching the shape of a bulb. Preferably the law of variation is chosen to obtain a connection by inflection and without rupture of an intermediate trunk of revolution at

  cylindrical central section of the preform.

  A nonlinear decay law allows for example to obtain a pointed end, joined to the central section by a curved junction of paraboloidal shape of revolution which bends, without breaking point, in the area where it

  laterally joins the central section.

  It is also possible to produce a cylindrical section 1 0 'which does not correspond to a perfect cylinder of revolution insofar as, for example, it is

slightly tapered.

  Whatever the case, it is necessary to separate the end piece 6b 'from the preform 1' obtained, to allow fiberizing. This separation is carried out in the preform zone, limited in length, which adjoins this endpiece and which composes the cylindrical section 10 'with it. Indeed, as it is known, the interface zone, where a tip is joined to a preform, is not used for fiberizing, because it is not sufficiently homogeneous due to the glass-glass welding by which the tip and

the preform are joined.

  The separation is carried out by stretching the glass constituting the section 1 0 ′ which is locally made soft by heart heating by means of the plasma torch 5, in the preform zone which comprises the section O10 ′ and which adjoins the end piece . This stretching is carried out by exerting axial pulls in the opposite direction by means of the end pieces 6a 'and 6b'. It leads to the sectioning of the link

  which existed between the preform 1 'and its end piece 6b', as shown diagrammatically in FIG. 3.

  This sectioning is facilitated by the location of the intensive heating, created by the core heating to soften the glass by fusion, in a portion of the section 10 ′ and by the fact that this section is of small diameter compared to the central section of

the preform 1 '.

  A more or less conical shape is obtained at the end of the preform, on the side where the rupture occurs with respect to the end piece 6b 'and to the preform residue 9' that

  this nozzle continues to support after separation.

  As indicated, the choice of the shape of the preform end best suited to fiberizing is obtained by acting on the factors involved in the stretch separation phase and in particular on the law of variation in length chosen for the superposed concentric layers of the junction between the section

  central cylindrical of the preform 1 'recharged and the section 10'.

  This makes it possible to avoid having to carry out the separation of a preform end having a diameter corresponding to that of the central section of the preform, when this diameter reaches the limit of value mentioned above from which the operations carried out at using a plasma torch become long and difficult to carry out. This limitation in diameter has the advantage of not involving

  limited tensile forces to perform the stretching operation.

  In addition, the duration of the separation operation is reduced since the diameter of the section 10 'is itself limited with respect to the overall diameter of the preform. This also has the consequence of limiting the amount of parasitic redeposit

  performed during core heating.

Claims (6)

  1. A method of manufacturing, or recharging, a preform for optical fiber in an installation provided with means allowing an axial rotation of a preform (1 ′), held horizontally at the ends between two mounting points (3a, 3b) by support end pieces (6a, 6b), and relative translations of the preform, held, said installation also being provided with preform heating means (5) by plasma torch, radially disposed relative to said preform and with which are associated material supply means, to allow manufacturing by successive passes corresponding to relative movements of the preform / torch, with or without material supply, these movements leading consequently, either to the deposition of a new layer of material on the preform , or to an icing on the last layer deposited, said method being characterized in that it intercalates at least one unilateral reduction in layer length, at the oc casion of a pass and from one of the intermediate layers, during the deposition of a series of concentric layers of material on the preform o the respective lengths of the layers, determined by the relative displacements preform / torch, are gradually reduced in length due to a progressive reduction in the length of the displacements, so that the thickness of deposited material, which covers the preform and partially the end pieces, is reduced regularly towards the ends, said unilateral reduction in layer length leading to a limitation of the thickness of material, deposited on one of the end pieces and for a preform zone, limited in length, which longitudinally adjoins this end piece, at the level fixed by the layer deposited prior to this reduction unilateral.   2. Method according to claim 1, characterized in that the unilateral reduction is carried out after deposition of a determined number of concentric layers   leading to a given preform diameter.   3. Method according to claim 2, characterized in that the given preform diameter, beyond which a unilateral reduction in layer length is carried out, is greater than the diameter of the endpiece concerned and less than 70 millimeters.   4. Method according to one of claims 1 to 3, characterized in that it provides for the   unilateral reduction in layer length, between 10 and 200 millimeters. 2791663   Method according to one of claims 1 to 4, characterized in that it provides for a   reduction of the layer lengths according to a linear law, at least beyond the layer unilaterally reduced in length which was carried out first and from   side of the preform where this reduction has been made.   6 Method according to one of claims 1 to 4, characterized in that it provides a   reduction of layer lengths according to a nonlinear decay law, at least beyond the unilaterally reduced length layer which has been   performed the first and on the preform side where this reduction was made.   7. Method according to one of claims 1 to 6, characterized in that it comprises at   minus a hot drawing operation carried out, to ensure a separation between a preform and one of the end pieces in said preform zone, limited in length, which adjoins this end piece, after completion of the series of layers   necessary for the formation of the preform.   8. Method according to one of claims l to 6, characterized in that it comprises   a hot drawing operation carried out in two stages, separated by a glazing operation of the preform, in said preform zone, limited in length, which adjoins an end piece to ensure separation between the preform and this end piece, the first step drawing being associated with a core heating causing a softening by fusion in said preform zone, limited in length, and producing a reduction in diameter, the second step also associated with a core heating causing a softening by fusion and ensuring complete separation.